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These two globular star clusters, M15 and G1, harbor hundreds of thousands of stars. But deep within their dense cores is an unexpected guest: a class of intermediate-sized black holes. Black holes are invisible, but the probing eye of NASA's Hubble Space Telescope found them by measuring the velocities of stars whirling around the crowded cores. Using spectral observations, astronomers discovered that the stars orbiting the cores of M15 and G1 moved at a much faster rate, which suggested the presence of unseen massive bodies. These previously undiscovered black holes provide an important link that sheds light on the way in which black holes grow.

The new findings promise a better understanding of how galaxies and globular clusters first formed billions of years ago. Globular star clusters contain the oldest stars in the universe. If these clusters have black holes now, then they most likely had black holes when they formed billions of years ago.

The black hole in M15 [left] is 4,000 times more massive than our Sun. G1 [right], a much larger globular cluster, harbors a heftier black hole, about 20,000 times more massive than our Sun.

The globular star cluster M15 resides 32,000 light-years away in the constellation Pegasus. M15 is one of nearly 150 known globular clusters that form a vast halo surrounding our Milky Way galaxy. G1, located 2.2 million light-years away in the neighboring Andromeda galaxy (also known as M31), has a total mass of 10 million suns, making it one of the most massive globular clusters known.

The Hubble telescope photograph of M15 was taken December 1998 by the Wide Field and Planetary Camera 2. Hubble's Wide Field and Planetary Camera 2 also snapped the image of G1, in July 1994.

The members of the G1 research team are Michael Rich (University of California, Los Angeles/UCLA), Karl Gebhardt (University of Texas at Austin), and Luis Ho (Carnegie Institute of Washington). The members of the M15 research team are Roeland Van Der Marel and Joris Gerssen (Space Telescope Science Institute), Karl Gebhardt, Puragra Guhathakurta and Ruth Peterson (UCO/Lick Observatory, University of California at Santa Cruz), and Carlton Pryor (Rutgers University).